The present invention concerns a novel method and apparatus for obtaining a desired rate of plasma collection from a membrane plasmapheresis filter.
In plasmapheresis, plasma is extracted from whole blood taken from a donor and the plasma is collected with the red cells being returned to the donor. Although extraction of plasma by centrifugation has been widely used, membrane plasmapheresis has recently been found to be very effective and more desirable in certain circumstances.
In membrane plasmapheresis, the whole blood is filtered through a microporous membrane having a pore size to pass the plasma filtrate but blocking the cellular material which remains on the upstream side of the microporous membrane. In abandoned U.S. patent application Ser. No. 942,077, filed Sept. 13, 1978, in the names of James H. DeVries, Ludwig Wolf, Jr., Gaylor Berry and William J. Schnell, and entitled "Apparatus for Membrane Plasmapheresis", a membrane plasmapheresis filter cell 12 is disclosed, as shown in FIGS. 2 and 3 of the instant application. The filter cell 12 includes a pair of generally parallel filter membranes 1 defining a blood flow path 2 therebetween, a pair of plasma filtrate volumes 3 defined on the opposite sides of the membranes 1 from the blood flow path 2, an outer casing 4 surrounding the membranes 1 and plates 5 supporting the membranes 1, a blood inlet 34 communicating with the blood flow path 2, a blood outlet 36 communicating with the blood flow path 2, and a plasma outlet 38 communicating with the plasma filtrate volumes 3. We have discovered that during plasmapheresis, the blood flow path gap 2 between the microporous membranes 1 tends to increase, thereby lowering the transmembrane blood pressure resulting in a decreased plasma collection rate. We have also discovered that an optimum plasma collection rate can be calculated by first determining the maximum whole blood flow rate desired (with the physical factors of the donor taken into consideration for this determination) then determining the inlet blood pressure to the membrane plasmapheresis filter cell which will result in this maximum whole blood flow rate, and then maintaining this inlet blood pressure during plasmapheresis to obtain a desired rate of plasma collection. As an example, a doctor would decide how much flow rate of blood he can safely obtain from a donor. The total whole blood flow rate is a combination of the blood taken from the donor and also the blood infused by recirculation. The doctor may set up a rate, for example, 250 ml per minute, and then by graphical determination, the doctor can determine what blood pressure at the filter cell inlet is necessary to obtain a desired plasma collection rate. Since the plasma collection rate is generally proportional to the inlet blood pressure at a particular whole blood flow rate, by increasing the inlet blood pressure a greater plasma collection rate can be achieved.
It is, therefore, an object of the present invention to provide a method for obtaining a desired rate of plasma collection, by maintaining the blood flow gap between the membranes.
Another object of the present invention is to provide a method for obtaining a desired plasma collection, using a pair of membranes and adjustable means to maintain the blood flow path gap.
A further object of the present invention is to provide apparatus for maintaining a desired blood film thickness between the membranes of a membrane plasmapheresis cell, by forcing the cell in a manner so that the film thickness remains constant as desired.
A still further object of the present invention is to provide a method and apparatus for obtaining a desired rate of plasma collection from a membrane plasmapheresis filter cell, by measuring the blood inlet pressure to the cell and maintaining the blood inlet pressure constant.
Other objects and advantages of the present invention will become apparent as the description proceeds.